warning: object backing the pointer will be destroyed at the end of the full-expression for std::pair

In std::string_view v = s.substr(i);, std::string::substr returns std::string by value, i.e. it returns a temporary std::string. std::string could be converted to std::string_view implicitly, which is constructed from std::string::data(), i.e. the pointer to the underlying array of the temporary std::string. After the full expression, the temporary std::string is destroyed and the pointer held by std::string_view becomes dangling pointer.

It is the programmer’s responsibility to ensure that the resulting string view does not outlive the string.

std::string get_string();
int f(std::string_view sv);
 
int x = f(get_string()); // OK
std::string_view sv = get_string(); // Bad: holds a dangling pointer

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